Avoid using dd(1) for raw writes to SSD. The SSD will think those blocks are in use. Without TRIM to tell it otherwise, they will always be marked in use until overwritten. Enough of that, and performance will suffer, possibly badly. As far as I know, the only way to fix that is with a “secure erase” which marks every block on the drive as free. At present, there is no easy way to do a secure erase on FreeBSD.

I would expect an 840 Pro to go faster than that. Even on SATA II, I’d hope for closer to the 300M/second limit. Certainly that drive is worth putting on a SATA III controller. For comparison, diskinfo says a 256G Plextor M3 does 405M/second raw reads on the SATA III controller on a Z68 motherboard.